During the last decade there is an ever lasting demand in medical diagnosis to get an image in quite a short time after the patient has been exposed to X-rays with preferably minimum radiation doses. An important step having a determining influence on the total time between exposure of the patient and examination by the radiologist is the processing time. Materials coated from emulsions having crystals rich in silver chloride are advantageous with respect to rapid processing (shorter developing times as well as fixation times for the said crystals) if compared with those coated from emulsions rich in silver bromide or silver bromoiodide (see e.g. EP-A 0 712 036 which is illustrative for mammographic applications) as has been demonstrated e.g. in EP-A 0 794 456. These references are illustrative for the feasibility in diverse applications of using small cubic grains rich in silver chloride having a crystal diameter of less than 0.65 .mu.m.
When images are generated from silver halide photographic film materials exposed to appropriate visible light for which the film materials are made sensitive, wherein said visible light is generated by conversion of X-ray irradiated intensifying screens held in intimate contact with said screens, subsequently followed by processing of the said film materials, then it is a stringent requirement to obtain a high covering power and a good image tone (color hue) of the developed silver, preferably a purely black image, in the already mentioned rapid processing conditions. The said image tone is closely related with the crystal size of cubic silver halide emulsions as becomes clear from e.g. EP-A 0 555 897. So it is well-known that in order to get a suitable "pure black" image tone, cubic grains having a lower sphere equivalent diameter are preferred.
It is further clear that in wet processing conditions chemical waste after processing of the said materials should preferably be reduced to minimum amounts. Therefore it is recommended to reduce replenishing amounts of developer and fixer. Especially when silver halide photographic materials are strongly hardened cross-over of liquid processing solutions and of rinsing water is reduced to a minimum and the drying time of the processed material is considerably reduced.
Otherwise as there is a demand for environmental friendly or ecologically justified systems for image formation in order to minimize the load of the environment at the level of the customer low coating amounts of silver halide in the silver halide material are preferred. However the said low coating amounts may lay burden on the preferred high covering power of the developed crystals. In EP-A 0 709 730 and in EP-Application No. 96203728, filed Dec. 30, 1996, it has further been shown that in a developer having an adapted chemical composition, covering power is advantageously increased.
Strongly hardened silver halide photographic materials moreover provide the advantage to use concentrated developing and fixing solutions free from hardening agents as has been set forth e.g. in U.S. Pat. No. 5,296,342, which again is in favour of ecology.
From the side of the manufacturer of silver halide photographic film materials, it is thus of utmost importance to provide strongly hardened films coated from low amounts of silver halide in favour of consumption of low amounts of chemicals, wherein said films can be processed in a processing cycle wherein hardener free processing solutions are used, without loosing speed or covering power in short (rapid) processing times. Efficient cross-linking of the gelatin chains of the photographic material indeed reduces the amount of water absorption in the processing cycle of the said material comprising cubic emulsion crystals having a grain diameter of less than 0.65 .mu.m.
An ever lasting demand however will remain to further improve the sensitivity (speed) of the said crystals from the side of crystal habit and/or composition on one hand and chemical and/or spectral sensitization at the other hand, especially under the severe limiting circumstances described above.
In order to get high speed it is well-known that use of crystals rich in silver bromide is feasible. Indeed from a more fundamental point of view silver bromide and silver chloride strongly differ in the energetical position of the highest valence energy level. Particularly silver halide crystals rich in chloride, spectrally sensitized with a green, red or even infrared dye, show a limited quantum efficiency due to instable dye holes (electron vacancies) on the silver halide crystal surface, leading to recombination between photo-electrons and dye positive holes. Especially for high coverage degrees of one or more dyes, it is required to enhance absorption of light emitted from phosphors present in intensifying screens at the crystal surface as "desensitization" is a factor limiting sensitivity. In order to overcome recombination the most suitable way consists in adding of a reducing agent, also called "supersensitizer", which may however become a competitive adsorbing compound for the sensitizing dye, limiting thereby its "maximum dye coverage". Moreover the presence of such organic dye compounds in amounts of mmoles per mole of silver is disadvantageous from the point of view of residual coloration of the processed material and coloration of processing liquids. Another disadvantage introduced by addition of such reducing agent is the generation of fog and poor preservability of the coated materials.